Enameling steel sheet has long been used extensively as a material for kitchen equipment such as pots, pans, kettles and sinks, building materials, and the like. Enameling steel sheet has conventionally been produced by ingot casting into capped steel or rimmed steel, blooming, hot rolling and cold rolling, followed by decarburization by open coil annealing and then denitrification annealing to reduce carbon and nitrogen content to several tens of ppm or less. However, the enameling steel sheet produced in this manner has the disadvantage of high production cost because it is produced by ingot casting and blooming and also requires decarburization and denitrification annealing. Another problem is that it cannot be applied to components that require forming by intensive deep drawing.
Recent enameling steel sheet is therefore usually produced by the continuous casting method so as to reduce production cost. In addition, the chemical composition is controlled by inclusion of various addition elements so as to simultaneously achieve good formability and enamelability. To give an example, it is known that Nb and V, for instance, enable production of enameling steel sheet having good formability and enamelability (see, for example, Japanese Patent No. 2040437 and Japanese Patent No. 3435035). This prior art method is low in deoxidation capability and therefore enables the oxygen content of the steel to be kept high and, moreover, is a breakthrough technology in that it adds Nb and V as elements capable of imparting good formability by immobilizing C and N present in the steel as carbide and nitride. Further, the prior art also teaches enameling steel sheet added with Cr and Nb to obtain a product that maintains good formability while resisting softening during firing (see, for example, Japanese Patent Publication (A) No. H11-6031) and enameling steel sheet technology that while unrelated to enamelability and formability, is added with Nb and V for avoiding swelling that under special circumstances may occur situation-specifically during casting when Sn is added (see, for example, Japanese Patent No. 3111834).
Further, the inventors sought to improve Nb- and V-containing enameling steel sheet excellent in fishscale resistance and deep drawability and filed earlier applications (see Japanese Patent Publication (A) No. 2002-249850 and No. 2004-84011). The essence of these technologies lies in controlling oxide form by giving consideration not only to Mn, conventionally the main oxide control element of enameling steel sheet, but also to Al, Nb, V, Si and the like in addition to Mn. Particularly noteworthy is Japanese Patent Publication (A) No. 2002-249850, which includes an unprecedented feature in that it takes the hot rolling conditions into account, with consideration being extended up to change in oxide shape caused by the rolling, so as to incorporate optimum characteristics. The steel sheets according to these techniques achieve stable and high r value and good fishscale resistance, and despite the increase in product cost owing to use of Nb, V and other expensive elements, are being used in increasing amounts chiefly in the high-grade material market.
However, with the recent polarization in use of steel sheet, that is, the use of low cost materials as much as possible for general purpose products and, on the other hand, materials with characteristics superior to those in the past for high grade products, these materials are being required to offer still better formability and enamelability. In particular, a very strong need is felt for further improvement of fishscale resistance, which can be said to be the most salient feature of enameling steel. It is known that in enameling steel sheet, fishscale can be effectively inhibited by forming voids in the steel sheet and trapping in the voids hydrogen entering the steel sheet during firing of the enamel. However, mere formation of voids does not improve the hydrogen trapping ability but, as can be seen from Japanese Patent Publication (A) No. H11-6031 and Japanese Patent No. 3111834, for example, the effect of controlling the form of the oxides is not clear. Viewed in this regard, it cannot be said that even Japanese Patent Publication (A) No. H11-6031 and Japanese Patent No. 3111834 achieve optimum control in the aspects of void volume, form and property, so that there can be considered to be a possibility for further improvement of fishscale resistance by controlling the steel composition and the oxides present in the steel.